For quality control purposes, it is desirable that commercially sold industrial bobbins carrying strand or yarn (hereinafter referred to simply as "strand") made up from multiple individual fibers or filaments of about the same average diameter to also contain thereon the same length of strand. This insures that any variation in the length of strand supplied from a group of randomly selected bobbins is as small as possible. This is important in the manufacture of fabricated products by customers who use strand supplied on bobbins to make beams for the production of woven fabrics for example. Since several hundred bobbins may be used in making a single beam, the early run-out of one bobbin will interrupt the production process while strand supplied from a new bobbin is spliced in. Such interruptions are obviously inefficient and costly to a fabric manufacturer.
In the industrial production of textile strand, it has been a common practice to wind strand onto as many as 100 or more bobbins using a single apparatus known as a twist-frame. Each bobbin is fed from a single forming package of strand placed on an individual feed roller or creel. The creels are often driven by means of a common drive mechanism from which they may be independently disengaged so as to cease rotating. Other twist-frames may have creels that are individually driven. Likewise, the rotating bobbins may be driven individually or through a common drive train. As strand is unwound from each forming package and onto a bobbin, a twist is imparted by means of a traveler ring which also rotates about the bobbin while simultaneously traversing up and down along its length.
One method which has been used in the past to measure the length of strand wound onto the bobbin was to simply count the number of revolutions of the forming package or bobbin mechanically by an odometer-type of device. This approach, while simplistically attractive, is subject to significant errors since the effect of forming package shape and the radially increasing buildup of yarn on the bobbin is not taken into account. A second method of insuring that each bobbin carries the same length of yarn is to simply run paired forming packages and bobbins for a fixed period of time and rely upon the consistency of the strand from each forming package to produce bobbins having uniform lengths of strand thereon. Still yet, another method by which to more accurately measure the length of strand wound onto the bobbin is simply to bring the moving strand in contact with the periphery of a rotatable disc or rim of known diameter. As the strand advances, the tractive force between it and the surface of the rim will cause the rim to rotate as long as contact is maintained. Ideally, the tangential velocity of the rim will equal the lineal velocity of the advancing strand and, therefore, each revolution of the rim will measure off a length of strand equal to its circumference. By counting the number of revolutions of the rim and then multiplying by its circumference, one can theoretically determine the total length of strand advanced past the rim and onto the bobbin. This was the basic concept used in my earlier U.S. patent application Ser. No. 07/331,765 which disclosed the use of an air bearing to support a rotatable metering rim.
In the case of fine textile strands, particularly those made from a plurality of individual glass fibers, several problems must still be overcome. Usually, it is necessary to maintain a low axial tension in the strand to prevent its breakage and other adverse effects during the twisting process. Nevertheless, broken filaments are often present in the strand and they may tend to break off from it and wrap themselves about the rim, buildup about the circumference thereof, and eventually impede the rotation of the metering rim.
As will now become apparent from the remainder of this disclosure, the instant invention minimizes the aforementioned problems.